A parking brake module of the general type under consideration is described in DE 103 36 611 A1, in particular FIG. 8. This module has a plurality of valves or valve devices that can mainly be activated electrically. On the one hand, the parking brake module is connected to a compressed air reservoir vessel. On the other hand, a first compressed air line leads to a spring-loaded component of a spring-type brake cylinder embodied as a combined spring-type/diaphragm brake cylinder. Such combined brake cylinders have both the function of diaphragm brake cylinders and a spring-loading function. The brake cylinders each include a diaphragm component that is pneumatically connected to the service brake system and to which the actual brake pressure can be applied. In addition, such brake cylinders each include a spring-loaded component that is pneumatically disconnected from the diaphragm component and to which compressed air can be applied via separate compressed air lines.
The spring-loaded component implements the spring-loading function by virtue of the fact that when compressed air is applied to the spring-loaded component a pre-loaded spring is pre-stressed and, in the process, a braking effect of the spring energy stores is reduced. When the spring-loaded component is vented, the pre-loaded spring is relaxed so that, in the scope of the spring-loading function, a braking effect is applied to the brake that is connected to the respective brake cylinder. Brake cylinders of this type are referred to hereinafter as spring-type brake cylinders. Such spring-type brake cylinders enable a parking brake function that permits the vehicle to be braked or arrested even when there is a lack of compressed air.
The above-mentioned parking brake module has a further connection to a trailer control valve. The trailer control valve is used to control the brake pressure of an attached trailer vehicle; and, in this context, this should also be understood to apply to a semitrailer. The trailer control valve has a port for a control line of the trailer service brake that leads to the trailer vehicle. The trailer control valve inverts the pressure applied to its inlet. That is, if a high pressure coming from the service brake module is present at its input, a low service brake pressure is modulated, and vice-versa.
The parking brake module has a first valve device, specifically a relay valve, and a second valve device, a pressure-holding valve and a bistable valve. The relay valve includes an inlet that can be connected to a compressed air reservoir vessel. The relay valve further includes an outlet that can be connected to at least one spring-loaded component of a spring-type brake cylinder. The relay valve also includes a control inlet. The pressure applied to the control inlet is modulated at the outlet of the relay valve, but with a significantly larger quantity of air. In this way, the necessary volumes for activating the spring-type brake cylinders can be made available.
The control pressure applied to the control inlet of the relay valve can be set using a pressure-holding valve and a bistable valve. In this way, the pressure at the outlet of the relay valve can also be set.
The second valve device includes a 3/2 way solenoid valve. A first port of this 3/2 way solenoid valve leads to the line that leads to the trailer control valve. A second port of this valve is connected to the outlet of the relay valve. A third port can be connected to the compressed air reservoir vessel. In a de-energized state, this 3/2 way solenoid valve connects the first port to the second port, while the first port is shut off from the third port. In contrast, in an energized state, the first port is shut off from the second port and the first port is connected to the third port.
In an energized state of this 3/2 way solenoid valve, the trailer control valve is therefore vented, and, in this way, the trailer brake is released. Venting the trailer control valve at this port leads, specifically, to a reduction in the service brake pressure at the outlet of the trailer control valve, with the result that the trailer brake pressure decreases and the trailer brake is, thus, released.
However, in a de-energized state, the pressure in the trailer control valve is the same as the pressure at the outlet of the relay valve, and, therefore, identical to the pressure in the spring-loaded component of a spring-type brake cylinder.
However, the conventional pneumatic circuit arrangement described above leads to a situation in which it is not possible to brake the trailer while the towing vehicle is unbraked. With the conventional parking brake module, it is, therefore, not possible to implement what is referred to as a trailer safety brake function.
Such parking brake systems are used in different vehicle control systems such as, for example, electronic brake systems, anti-lock brake systems or systems without anti-lock brake protection. The parking brake module has to be adapted to these different vehicle control systems. It is, therefore, manufactured in a wide variety of designs. This reduces the respective number of a specific type of parking brake module. However, a reduced number increases the costs of manufacture, administration and stock keeping.